I have cancelled occasionally because of work committments but that is usually a week in advance. I have also cancelled for cancer surgeries again, well in advance. My T has a 24 hour notice or I would have to pay. I have cancelled about 3-4 times when he has done something that upset me in a session and I didn't want to see him or talk to him. I needed time to cool off and think through my feelings. In those cases I will send him an email usually the day after the session and tell them I need to cancel for the next session. I will usually not give a reason for those times.

That is me. In answering the broad question, yes. People cancel all of the time. I'm sure each practice is different depending on the clientelle but it is a problem that T need to deal with. That is why they have their own cancellation policies and most will charge without adequate notice.